Now Graham is going after Trump in what is perhaps his most vulnerable spot — those tax returns that he refuses to release.

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"I think that prospectively what I'll do is, any candidate running in 2020 needs to release their tax returns," Graham told reporters on Wednesday. Although he couldn't retroactively force this on Trump for the 2016 presidential election, Graham has a simple way to do so for the upcoming contest in 2020: "You just make it a law."

Graham's proposal would not specifically target Trump, but rather require all future presidential candidates to release their tax returns.

Graham's tightening of Trump's tax screws puts him in the same league as another Republican, Senator Susan Collins of Maine, who last week suggested that she would be open to subpoenaing Trump's tax returns as part of the Senate Intelligence Committee's investigation into Russia's alleged hacking of the 2016 presidential election.

It was revealed during the 2016 presidential election that Trump had managed to manipulate a tax-saving trick known as a "stock-for-debt swap" in order to save himself hundreds of millions of dollars in taxes. This explained the $916 million of losses that Trump reported in his 1995 tax returns, which were reported by The New York Times in October.

Matthew Rozsa

Matthew Rozsa is a breaking news writer for Salon. He holds an MA in History from Rutgers University-Newark and is ABD in his PhD program in History at Lehigh University. His work has appeared in Mic, Quartz and MSNBC.